The present invention relates to optical amplifiers for fiber optic networks, and, more particularly, to a dynamically tunable optical amplifier.
The present invention also relates to a light source, and, more particularly, to a tunable fiber light source.
Optical networks in which the signals are wavelength division multiplexed (xe2x80x9cWDMxe2x80x9d) require a uniform spectral distribution across all channels. As an optical signal traverses an optical network, however, the signal is subject to losses and nonlinear effects that result in signal attenuation and distortion. Therefore, amplifiers, such as erbium-doped fiber amplifiers (xe2x80x9cEDFA""sxe2x80x9d), are typically placed approximately every 80 kilometers along an optical fiber to boost signal strength. However, such an amplifier has a non-uniform gain profile (as a function of wavelength), which, in turn, imposes a distortion on the signal power spectral distribution (as a function of wavelength). The gain profile of the optical amplifier changes as a function of the input power, as well. Optical amplifiers based on the Raman effect also exhibit spectral gain variations.
Static gain flattening filters (xe2x80x9cSGFF""sxe2x80x9d) are often used to attenuate the signal power as a function of wavelength to achieve a substantially uniform power distribution. Static filters (otherwise known as passive filters), however, cannot adapt to dynamically changing conditions such as amplifier aging, temperature variations, channel add/drop, fiber loss, amplifier gain profile variations due to input power variations, and other changes in components along the transmission line. Moreover, the required filter shape is dependent upon system configuration, e.g., the spacing between amplifiers. Static filter characteristics cannot be modified to compensate for these changes without replacing the filter itself.
An automatic gain control (xe2x80x9cAGCxe2x80x9d) optical amplifier employs an SGFF, in conjunction with a variable optical attenuator (xe2x80x9cVOAxe2x80x9d), to achieve a constant gain profile over a wide spectrum. In these amplifiers, the output power per channel is proportional to optical input power, since the gain of the amplifier is a constant regardless of the input power to the amplifier.
FIG. 1 illustrates a typical configuration of an AGC amplifier 100 for WDM transmission systems and networks. It is a two-stage optical amplifier comprising, for example, two erbium-doped fiber amplifiers (xe2x80x9cEDFAxe2x80x9d) EDFA1102 and EDFA2104. The first amplifier stage 102 (i.e., xe2x80x9cgain stagexe2x80x9d) may be considered a pre-amplifier and the second 104 a post-amplifier. Those skilled in the art will recognize that other types of optical amplifiers known in the art, e.g., semiconductor optical amplifiers or Raman amplifiers, may also be used for the amplifier stages. Moreover, as is well known in the art, EDFA""s and other amplifiers typically incorporate a pump laser (not shown). These amplifier stages have spectral transfer functions (otherwise known as transmission vs. wavelength profiles, gain profiles, or gain curves, among other terms) that are not uniform as a function of wavelength.
A static gain flattening filter 106, a variable optical attenuator 108, and a dispersion compensating fiber (xe2x80x9cDCFxe2x80x9d) 110 are located between the two amplifier stages. Each stage of the optical amplifier provides a constant gain regardless of the input power to the amplifier. The constant gain can be maintained by changing the pump power for each amplifier stage as a function of the input power. Typically, the higher the input power, the higher the pump power that is required to maintain a constant gain. A higher pump power, however, causes a higher noise level within the amplifier stage.
At a particular gain level, each amplifier stage has a nonuniform spectral gain profile. The static gain flattening filter 106 is used to flatten the gain of the amplifier using a fixed gain profile based on the amplifier stages"" known nonuniform gain profiles. The gain of each stage is kept constant because varying the gain would change the spectral profile, thereby requiring a different SGFF. Instead of varying the gain of the amplifier stages to maintain a constant output power per channel, a controller 112 of the AGC amplifier 100 varies the attenuation level of the VOA 108 to adjust the gain of the entire amplifier. The VOA attenuation level decreases/increases, when the input power decreases/increases. In addition, the optical power into the DCF 110 should be less than xe2x88x923 dBm/ch to avoid signal distortion due to optical nonlinearities in the DCF 110.
Although the AGC amplifier 100 can provide a constant output power for a wide range of input power, a drawback of the AGC amplifier is the large noise generated in the amplifier. To describe the loss contributors, consider, for example, an amplifier that has a range of gains from 16 dB (compensating an 80 km span of optical fiber having 0.2 dB/km loss) to 28 dB (compensating a 100 km span of optical fiber having 0.28 dB/km loss). The amplifier is designed to have a maximum 28 dB gain. The gain of the first stage amplifier 102, the insertion loss of the static gain flattening filter 106, the loss of the variable attenuator 108, the loss of the DCF 110, and the gain of the second stage amplifier 104 are 15 dB, 1 dB, 2 dB, 7 dB, and 23 dB, respectively.
FIG. 2 illustrates the power levels along the amplifier module 100. For a maximum gain of 28 dB, if one assumes that the output power per channel required of the amplifier is 5 dBm/ch, the input power of the first stage amplifier 102 is xe2x88x9223 dBm/ch, and that of the second stage amplifier 104 is xe2x88x9218 dBm/ch. Because the input power into the first stage amplifier 102 is much smaller that that of the second stage amplifier 104, the noise of the first stage amplifier 102 is dominant with respect to the output noise.
On the other hand, if the input power is xe2x88x9211 dBm/channel, then a gain of 16 dB is required to achieve the same output power per channel of 5 dBm/ch. In this case, the input power of the first stage amplifier 102 is xe2x88x9223 dBm/ch, and that of the second stage amplifier 104 is xe2x88x9218 dBm/ch. Because the gain of each amplifier stage is constant regardless of input power, the AGC amplifier requires that the loss of the variable attenuator 108 be increased to 14 dB to achieve the same output power. In this case, the noise of the amplifier is dominated by the noise generated from the second stage 104, if one assumes the same noise figure for the two amplifiers. In other words, the contribution of the second stage amplifier 104 to the output noise power is five times that of the first stage amplifier 102. Thus, the AGC amplifier provides a high noise figure when it is operated to have a wide gain dynamic range. The origin of the high noise figure is the large amount of attenuation induced by the variable optical attenuator 108 and the DCF 110. Note that the input power to the DCF 110 is xe2x88x9211 dBm/ch regardless of the input power to the optical amplifier.
The gain dynamic range of the amplifier is equal to the input power dynamic range, since the output power of the amplifier is desired to be constant. If one specifies an amplifier noise figure, the dynamic range of the AGC amplifier is limited. In other words, it is better to design a different amplifier for 16 dB gain, instead of using the amplifier designed for 28 dB gain with a 12 dB dynamic range. More generally, a different design of the AGC amplifier for a different gain is required for satisfactory noise performance.
It would be advantageous to provide an optical amplifier that can maintain a desired output power per channel and a desired gain profile over a wide dynamic range. It would also be advantageous to provide an optical fiber light source that can produce a desired output spectrum using similar technology.
The present invention provides an optical amplifier comprising a wavelength tunable filter, one or more optical gain stages, and a controller for controlling a spectral profile of the wavelength tunable filter in response to a measured spectral characteristic of the amplifier. The controller may also control gain of the gain stage(s). The controller determines the filter spectral profile necessary to obtain a desired amplifier spectral characteristic. The spectral characteristic may, for example, be a power spectral output of the amplifier or a gain profile of the amplifier. A monitor measures the spectral characteristic of the amplifier.
The wavelength tunable filter may include an acoustic wave exciter and an optical fiber having an interaction region. The acoustic wave exciter induces an acoustic wave in the interaction region to couple light between a first mode and a second mode, e.g., between core and cladding modes. The amplifier may also include a fixed optical filter having a fixed spectral profile.
In another embodiment, the amplifier may incorporate a dispersion compensator. The controller may control a spectral profile of the wavelength tunable filter and gain of the dispersion compensator. In addition, the controller may control gain of the gain stage(s), as well. The dispersion compensator may be a dispersion compensating fiber coupled to a pump laser, which is controlled by the controller.
According to similar principles, a fiber light source may include an optical fiber having a doped gain medium. An optical pump coupled to the fiber generates an amplified spontaneous emission in the fiber. A wavelength tunable filter controls the spectral shape of the amplified spontaneous emission.